1. Field of the Invention
This invention relates to a voltage nonlinear resistor which is made of a sintered substance consisting essentially of zinc oxide and can be used preferably for a lightning arrester, a surge absorber, etc., for example, and a lightning arrester having the voltage nonlinear resistor mounted thereon.
2.Description of the Prior Art
FIG. 10 is a schematic diagram to show the structure of a general zinc oxide varistor. Hitherto, a voltage nonlinear resistor consisting essentially of zinc oxide, used for a lightning arrester, etc., has been manufactured as follows. Compositions comprising additives effective for improvement of electric characteristics including bismuth oxide indispensable for development of voltage nonlinearity added to zinc oxide of an essential component are mixed, granulated, molded, and sintered to provide a sintered substance and electrodes made up of side face high resistance layer, metal aluminum, etc., are placed on the sintered substance.
FIG. 11 is a schematic diagram to show the microstructure of a part of the crystalline structure of a general voltage nonlinear resistor. Numeral 1 is a spinel grain consisting essentially of zinc and antimony, numeral 2 is a zinc oxide grain, numeral 3 is zinc silicate Zn.sub.2 SiO.sub.4, numeral 4 is oxide bismuth, and numeral 6 is a twin boundary in a zinc oxide grain. That is, the spinel grains consisting essentially of zinc and antimony are classified into two types of those surrounded by the zinc oxide grains and those existing in the vicinity of the triple point (multiple point) of the zinc oxide grains, and a part of the bismuth oxide 4 exists not only at the multiple point, but also on the boundary of the zinc oxide grain 2.
An experiment using point electrodes reveals that the grains consisting essentially of zinc oxide serve simply as a resistor and show voltage nonlinearity on the boundary between the zinc oxide grains 2 and 2 (G. D. Mahan, L. M. Levinson & H. R. Philipp, "Theory of conduction in ZnO varistors," (J. Appl. Phys. 50[4], 2799 (1979), which will be hereinafter referred to as document 1. As described later, it is acknowledged by experiment that the number of boundaries between the zinc oxide grains 2 and 2 (grain boundaries) (T. K. Gupta, "Application of Zinc Oxide Varistors," J. Am. Ceram. Soc., 73[7]1817-1840 (1990), which will be hereinafter referred to as document 2.
FIG. 12 is a volt-ampere plot to show the voltage-current characteristic (nonlinear characteristic) of the general voltage nonlinear resistor having the crystalline structure. A zinc oxide family voltage nonlinear resistor having excellent protection performance has a small ratio between voltage V.sub.H in large current area H and voltage V.sub.L in small current area L, V.sub.H /V.sub.L (discharge voltage ratio) in the figure. To discuss improvement in the discharge voltage ratio, factors determining the discharge voltage ratio in a large current area and that in a small current area differ, thus the discharge voltage ratios need to be discussed separately. Therefore, in the description that follows, voltage V.sub.S in S in the figure is used and the discharge voltage ratio in the large current area, V.sub.H /V.sub.S, and that in the small current area, V.sub.S /V.sub.L, will be discussed separately.
V.sub.H of the discharge voltage ratio in the large current area, V.sub.H /V.sub.S, is determined by electric resistivity in zinc oxide crystal grains (documents 1 and 2). The smaller the resistivity in zinc oxide crystal grains, the smaller V.sub.H. Therefore, V.sub.H /V.sub.S lessens. On the other hand, the discharge voltage ratio in the small current area, V.sub.S /V.sub.L is determined by a Schottky barrier probably formed in the zinc oxide crystal grain boundary (documents 1 and 2). The larger the apparent resistivity of the zinc oxide crystal grain boundary, the smaller V.sub.S /V.sub.L. Therefore, to improve the discharge voltage ratio V.sub.H /V.sub.L, the electric resistivity in zinc oxide crystal grains needs to be reduced and the apparent electric resistivity of the zinc oxide crystal grain boundary needs to be raised.
In the voltage nonlinear resistor, V.sub.S shown in FIG. 12 represents a nonlinear threshold voltage. The V.sub.S value is set for a transmission system to which lightning arresters are applied. For V.sub.S interelectrode voltage across a device when the device is energized with 1 mA (V.sub.1 mA (V)) or the like is often used as a representative value. Considering the device size, the current value 1 mA corresponds to a current density of about 30-150 .mu.A/cm.sup.2. The V.sub.S value of a zinc oxide device is proportional to the thickness of the device.
With a lightning arrester used for high-voltage power transmission of, for example, UHV 100 million volts or the like, if devices of the same shape having a VS value equal to that of the conventional device are piled up, the number of series lamination layers increases. Resultantly, the lightning arrester becomes large and the series connection system becomes complicated, thus electric, thermal, and mechanical design problems increase. Therefore, if a device having a large VS value per unit length provided by dividing the V.sub.S value by the device thickness (for example, V.sub.1 mA/mm, called varistor voltage) can be used, the share voltage per device is raised, so that the number of series lamination layers of the device can be decreased and the problems can be solved.
The former study shows that the crystal grain diameter of the zinc oxide 2 in the crystalline structure of the device shown in FIG. 11 controls the V.sub.S value (document 2). A current area of about 1 mA is a nonlinear area in the volt-ampere plot shown in FIG. 12 and experimentally expression (1) holds. EQU V.sub.1 mA/mm =mm/D (1)
where k is a constant and D is an average particle diameter of zinc oxide. Therefore, 1/D is equivalent to the number N.sub.g of the crystal grain boundary between zinc oxide grains existing per unit length and expression (1) can be rewritten as expression (2) EQU V.sub.1 mA/mm =k'N.sub.g (2)
It is seen that the constant k' represents a varistor voltage per grain boundary of the zinc oxide device (document 2).
In summary, to provide a compact lightning arrester having an excellent protection property, (a) the discharge voltage ratio (V.sub.H /V.sub.L) is small as the electric characteristic of a voltage nonlinear resistor and (b) the varistor voltage is increased as the electric characteristic required for a voltage nonlinear resistor necessary to provide a compact lightning arrester. It is strongly required that the discharge voltage ratio (V.sub.H /V.sub.L) is set to a small value by improving the composition and manufacturing process of the voltage nonlinear resistor because the factor for determining the protection property of the lightning arrester is (a) and that the varistor voltage is set to a large value because the factor for determining the structure such as the size of the lightning arrester is mainly (b).